532 STANLEY C. BALL 



V. DISCUSSION 



In the preceding account of the development of Paravortex 

 gemelUpara Httle attention has been paid to the interpretation 

 given by Hallez ('09) of the processes of yolk absorption and the 

 origin of the ectoderm in P. cardii. 



He mamtams that, while the morula is being formed by the 

 cleavage of the egg, the membranes of the yolk-cells included 

 in the capsule disintegrate, but that a part of their nuclei re- 

 main prominent and active. Hallez finds these nuclei shortly 

 afterward lying in a clear plasma-like portion of the yolk which 

 has become separated from an eosin-staining constituent and 

 has assumed a peripheral position outside of and between the 

 embryos. On the other hand the 'eosinophile' portion of the 

 yolk separates, he believes, because its chemical and physical 

 properties are different from those of the cytoplasmic remainder. 

 As these properties become more pronounced, the central yolk 

 material splits off in the form of great drops in whose interior 

 he finds a variable number of vacuoles. He has figured several 

 nuclei, which are assumed to have been set free by the disinte- 

 grating yolk cells, lymg in the outer clear zone; others have 

 migrated inward between the embryos. As pointed out earlier 

 in this paper, he found that one of these nuclei became associ- 

 ated with each of the 'balles vitelline.' 



Even more remarkable is Hallez 's account of the developmen- 

 tal phenomena which immediately follow. Into each vitelline 

 sphere one of these migratory nuclei penetrates, together with a 

 small quantity of the cytoplasm which also was previously a 

 constituent of the vitellarium cells. At the same time the 

 material of these yolk spheres assumes a granular appearance. 



A little later these nuclei, he states, emigrate with their 

 associated cytoplasm from the 'balles vitellines.' Part of these 

 amoeboid cells are asserted then to mingle with the envelope 

 of plasma outside the embryo so as to form a 'syncytium' from 

 which a primary ectoderm soon differentiates. Other such cells 

 delaminate from the yolk spheres as additions to the ectoderm. 

 Into the formation of the latter now enter, as Hallez believes. 



